Surface acoustic wave filter device

Abstract

A resonant surface acoustic wave filter device comprises an inter-digital transducer that has a first electrode area that excites a surface acoustic wave and a second electrode area that excites a surface acoustic wave of inverted phase with respect to that excited in the first electrode area, and grating reflectors. By selecting appropriately a ratio of numbers of electrode pairs and a pitch of electrodes of the first area and the second area, band pass characteristics and impedance characteristics of the filter devices can be controlled.

What is claimed is:  
     
       1. A resonant surface acoustic wave filter device, comprising: 
       a piezoelectric substrate;  
       an inter-digital transducer that is formed on the piezoelectric substrate and has a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set approximately equal to a pitch of an electrode fingers array of the first electrode area to make frequency characteristics of a pass band of the filter device flat such that the second surface acoustic wave partially counteracts the first surface acoustic wave.  
     
     
       2. The resonant surface acoustic wave filter device as set forth in claim  1 , wherein a plurality of inter-digital transducers is formed between the reflectors. 
     
     
       3. A resonant surface acoustic wave filter device, comprising: 
       a piezoelectric substrate;  
       an inter-digital transducer that is formed on the piezoelectric substrate and has a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set larger than a pitch of an electrode fingers array of the first electrode area to increase attenuation at low frequency outside a pass band of the filter device such that the second surface acoustic wave counteracts the first surface acoustic wave.  
     
     
       4. The resonant surface acoustic wave filter device as set forth in claim  3 , wherein a plurality of inter-digital transducers is formed between the reflectors. 
     
     
       5. A resonant surface acoustic wave filter device, comprising: 
       a piezoelectric substrate;  
       an inter-digital transducer that is formed on the piezoelectric substrate and has a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set smaller than a pitch of an electrode fingers array of the first electrode area to increase attenuation at high frequency outside of a pass band of the filter device such that the second surface acoustic wave counteracts the first surface acoustic wave.  
     
     
       6. The resonant surface acoustic wave filter device as set forth in claim  5 , wherein a plurality of inter-digital transducers is formed between the reflectors. 
     
     
       7. A resonant surface acoustic wave filter device comprising: 
       a piezoelectric substrate; and  
       two or more stages of surface acoustic wave filters having an inter-digital transducer that is formed on the piezoelectric substrate and has a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set approximately equal to a pitch of an electrode fingers array of the first electrode area to make frequency characteristics of a pass band of the filter device flat such that the second surface acoustic wave partially counteracts the first surface acoustic wave, and  
       wherein the surface acoustic wave filters are connected with each other.  
     
     
       8. A resonant surface acoustic wave filter device comprising: 
       a piezoelectric substrate; and  
       two or more stages of surface acoustic wave filters having an inter-digital transducer that is formed on the piezoelectric substrate and has a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set larger than a pitch of an electrode fingers array of the first electrode area to increase attenuation at low frequency outside a pass band of the filter device such that the second surface acoustic wave counteracts the first surface acoustic wave, and wherein the surface acoustic wave filters are connected with each other.  
     
     
       9. A resonant surface acoustic wave filter device, comprising: 
       a piezoelectric substrate;  
       a plurality of connected resonant surface acoustic wave filters formed on the piezoelectric substrate, at least one of which has an inter-digital transducer having a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set approximately equal to a pitch of an electrode fingers array of the first electrode area to make frequency characteristics of a pass band of the filter device flat such that the second surface acoustic wave partially counteracts the first surface acoustic wave.  
     
     
       10. A resonant surface acoustic wave filter device, comprising: 
       a piezoelectric substrate;  
       a plurality of connected resonant surface acoustic wave filters formed on the piezoelectric substrate, at least one of which has an inter-digital transducer having a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed both sides of the interdigital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set larger than a pitch of an electrode fingers array of the first electrode area to increase attenuation at low frequency outside of a pass band of the filter device such that the second surface acoustic wave counteracts the first surface acoustic wave.  
     
     
       11. A resonant surface acoustic wave filter device, comprising: 
       a piezoelectric substrate;  
       a plurality of connected resonant surface acoustic wave filters formed on the piezoelectric substrate, at least one of which has an inter-digital transducer having a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waver that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set smaller than a pitch of an electrode fingers array of the first electrode area to increase attenuation at high frequency outside of a pass band of the filter device such that the second surface acoustic wave counteracts the first surface acoustic wave.  
     
     
       12. A resonant surface acoustic wave filter device comprising: 
       a piezoelectric substrate; and  
       two or more stages of surface acoustic wave filters having an inter-digital transducer that is formed on the piezoelectric substrate and has a first electrode area that excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave; and  
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the first and second surface acoustic waves that are excited by the inter-digital transducer,  
       wherein a pitch of an electrode fingers array of the second electrode area of the inter-digital transducer is set smaller than a pitch of an electrode fingers array of the first electrode area to increase attenuation at high frequency outside of a pass band of the filter device such that the second surface acoustic wave counteracts the first surface acoustic wave, and wherein the surface acoustic wave filters are connected with each other.  
     
     
       13. A resonant surface acoustic wave filter device, comprising: 
       a piezoelectric substrate;  
       a surface acoustic wave filter comprising an inter-digital transducer that has a first area where polarity alternates every adjacent electrode finger and a second area where an alternating arrangement of a first set of plural electrode fingers has one polarity and a second set of electrode fingers comprises at least one having the other polarity,  
       wherein a ratio Pb/Pa of an electrode fingers array pitch Pb in the second area to a pitch in the first area Pa is 0.95 or more and 1.05 or less, and the second area of the inter-digital transducer excites two surface acoustic waves that counteract each other.  
     
     
       14. The resonant surface acoustic wave filter device as set forth in claim  13 : 
       wherein a plurality of surface acoustic wave filters are connected.  
     
     
       15. The resonant surface acoustic wave filter device as set forth in claim  13 : 
       wherein the second area of the inter-digital transducer works as a reflector that has capacitance between the electrode fingers disposed adjacent having different polarity.  
     
     
       16. The resonant surface acoustic wave filter device as set forth in claim  13 : 
       wherein the second area of the inter-digital transducer comprises two or more sequence of electrode fingers having same polarity.  
     
     
       17. The resonant surface accustic wave filter device as set forth in claim  13 , further comprising: 
       grating reflectors formed at both sides of the inter-digital transducer along a direction of propagation of the surface acoustic wave that the inter-digital transducer excites.  
     
     
       18. The resonant surface accostic wave filter device as set forth in claim  17 : 
       wherein a plurality of the inter-digital transducers are formed between the reflectors.  
     
     
       19. The resonant surface acoustic wave filter device as set forth in claim  13 : 
       wherein both widths and the pitches of the electrode fingers that the inter-digital transducer comprises are approximately a quarter of a wavelength of the surface acoustic wave that the inter-digital transducer excites.